Support new "Go to Source Definition" feature in TS 4.7

[RedBeard0531]

The readme for this repo mentions that goto definition does always work correctly and often drops you into a .d.ts file rather than the actual definition. It looks like they added the ability to go to a real definition in 4.7, but unfortunately as a new custom command, rather than as the default goToDefinition behavior. It would be nice if there was some way to access this functionality from coc-tsserver. If there already is, I couldn't figure it out 😄.

More info:
Blog announcement
Feedback Thread
Implementation PR